Method of depositing a thin film using an optical pyrometer

ABSTRACT

A temperature measurement system for use in a thin film deposition system is based on optical pyrometry on the backside of the deposition substrate. The backside of the deposition substrate is viewed through a channel formed in the susceptor of the deposition system. Radiation from the backside of the deposition substrate passes through an infrared window and to an infrared detector. The signal output by the infrared detector is coupled to electronics for calculating the temperature of the deposition substrate in accordance with blackbody radiation equations. A tube-like lightguide shields the infrared detector from background radiation produced by the heated susceptor.

This is a continuation of application Ser. No. 08/461,076 filed on Jun.5, 1995, now abandoned, which is a division of application Ser. No.08/190,421, filed Feb. 2, 1994, now U.S. Pat. No. 5,549,756.

FIELD OF THE INVENTION

The present invention is directed to the field of thin film depositionsystems.

BACKGROUND OF THE INVENTION

Modern liquid crystal displays, such as the active matrix liquid crystaldisplays used in some portable computers, are formed by the depositionof thin films on a glass substrate. These thin films are subsequentlyprocessed using standard photolithography techniques to form thecircuitry which drives the liquid crystal display. This circuitrytypically includes amorphous silicon field effect transistors and isformed directly on the surface of the glass substrate to optimize theintegration of these displays. The satisfactory performance of theresulting liquid crystal displays is dependent on the uniformity oftransistor characteristics, across the surface of the glass substrateand on the level of performance exhibited by individual transistorelements. Deposition of thin films of semiconductors, insulators andmetals on glass substrates is highly sensitive to the temperature of theglass deposition substrate. For example, the deposition temperaturestrongly affects the mobility of carriers in deposited amorphous siliconfilms and thereby affects the performance of MOSFETs formed from thedeposited amorphous silicon films. Accordingly, it is important tomaintain the deposition substrate at an appropriate temperature, as wellas to maintain a uniform temperature across the surface of thedeposition substrate.

As is typical of vacuum deposition systems, glass deposition substratesare loaded into a thin film deposition system through a vacuum loadlock. Because of the sensitivity of the thin film deposition process tothe temperature of the deposition substrate, a number of precautionarysteps are taken to ensure that the glass substrate is brought to theappropriate temperature before deposition begins. After a batch of glasssubstrates is admitted to the deposition system, the substrates areheated in a preheat chamber until they reach a temperature near thetarget deposition temperature. Once the substrates have preheated to asufficient degree, one of the substrates is transferred to thedeposition chamber by a substrate transfer robot in preparation for thinfilm deposition. Even if the transfer operation is performed quickly,the glass substrate will cool markedly during the transfer operation.Thus, the substrate must be preheated again to ensure that the substratehas uniformly obtained the appropriate deposition temperature.

The duration of this second preheat step is referred to as the "soak"time. The deposition substrate must be heated for a sufficient time toensure that the deposited films will have the desired characteristics,otherwise there is a risk that devices formed using the deposited filmswill have poor performance characteristics. It is thus desirable topreheat the substrate for sufficient time to ensure that the substratehas attained the desired deposition temperature. The soak time directlyimpacts the possible throughput of a given thin film deposition systembecause deposition cannot take place during the soak time. It istherefore desirable to optimize the soak time so that the second preheatstep takes only as long as is necessary to ensure that the substrate isat the appropriate temperature.

While it is highly desirable to monitor the temperature of thedeposition substrate in situ to establish when the deposition substratehas reached an appropriate deposition temperature, it is very difficultto accurately measure the temperature of a substrate within thedeposition chamber. For example, it is typically unsatisfactory tomeasure the temperature of the susceptor on which the substrate restsduring deposition. The pressure in the deposition chamber is typicallyabout 5 Torr, which means that, even when the substrate is in physicalcontact with the susceptor, there is too little gas conduction betweenthe deposition substrate and the susceptor to keep the substrate and thesusceptor in good thermal contact. The temperature of the susceptor istherefore a poor measure of the temperature of the deposition substrate.Similarly, a thermometer mounted adjacent to the substrate would not bein sufficient thermal contact with the substrate to accurately measurethe temperature of the substrate.

Pyrometry has been used to monitor the temperature of semiconductorwafers during processing. U.S. patent application Ser. No. 08/021,840,now abandoned entitled "Measuring Wafer Temperatures," lists ChristianGronet and Gary Miner as inventors and is assigned to the assignee ofthe present invention. The Gronet application describes a pyrometrysystem for measuring the temperatures of semiconductor wafers duringprocessing. In particular, the Gronet pyrometer is designed formeasuring the temperature of semiconductor wafers having unpredictableor low emissivities. A cavity is formed adjacent to the wafer whosetemperature is to be measured and blackbody radiation within this cavityis sampled to yield information about the temperature of the wafer. Thepyrometer described in the Gronet application is ill-suited to measurethe temperature of glass substrates in thin film deposition systemsbecause no cavity can be formed adjacent to the glass substrate withoutdiminishing the quality of thin films formed on the glass substrate. Nocavity can be formed on the side of the substrate that is away from thesusceptor because thin films must be deposited over that entire surface.Forming a cavity on the susceptor side of the glass substrate isunacceptable because it is likely that temperature of the glasssubstrate would drop slightly in the region adjacent to the cavity. Evenslight variations in the temperature of the glass deposition substrateare typically undesirable for thin film deposition systems. Moreover,the blackbody radiation associated with the susceptor itself is likelyto dominate the blackbody radiation formed in a cavity disposed on thesusceptor side of the deposition substrate.

SUMMARY OF THE PREFERRED EMBODIMENTS

In accordance with one aspect of the present invention, a thin filmdeposition system comprises a susceptor for supporting a depositionsubstrate during a deposition process. A channel is formed in thesusceptor, with the channel having an opening on the upper surface ofthe susceptor. An infrared detector is disposed to receive an opticalsignal that passes through the channel along an optical path from theopening to the infrared detector.

In accordance with a further aspect of the present invention, the outputsignal from the infrared detector is coupled to electronics capable ofcalculating the temperature of a blackbody on the basis of theblackbody's radiation characteristics.

In accordance with a further aspect of the present invention, thedeposition system includes a hollow lightguide extending from thesusceptor and defining an optical path to the infrared detector.

In accordance with another aspect of the present invention, a method ofdepositing thin films includes the steps of transporting a depositionsubstrate to the surface of a heated susceptor and heating thedeposition substrate to a target temperature. Radiation from thedeposition substrate is collected and the apparent temperature of thedeposition substrate is determined from the infrared radiation emittedby the deposition substrate.

In accordance with a further aspect of the present invention, the abovemethod can include the further steps of transporting a cold substrate tothe surface of the susceptor and measuring a level of backgroundradiation before the cold deposition substrate is substantially heated.A background reference signal is stored and subsequent temperaturemeasurements are made relative to the background reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a thin film deposition system in accordance with thepresent invention.

FIG. 2 illustrates an expanded view of a section of the thin filmdeposition system of FIG. 1.

FIG. 3 illustrates an expanded view of a section of the thin filmdeposition system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention utilize optical pyrometryto measure the temperature of a deposition substrate within thedeposition chamber of a thin film deposition system. For example, aninfrared detector may be disposed so as to image a small portion of thebackside of a deposition substrate within the deposition chamber. Alightguide leading from the backside of the deposition substrate towardthe infrared detector acts as a shield to reduce the level of backgroundradiation that is incident into the infrared detector. In particularlypreferred embodiments, the intensity of the infrared radiation receivedby the infrared detector is measured, and the temperature of thedeposition substrate is determined from the blackbody radiation emittedby the backside of the deposition substrate. The output from theinfrared detector may be coupled into the control system of the thinfilm deposition system so that the system may utilize the temperatureinformation to determine when a deposition process can begin.

Optical pyrometry is based on the principle that materials of a giventemperature radiate energy in accordance with Planck's law: ##EQU1##wherein "ν" (the greek letter nu) represents frequency, "h" is Planck'sconstant, "c" represents the speed of light, and "T" is the absolutetemperature. Planck's law sets forth as a function of temperature theamount of energy radiated at a given frequency by a perfect blackbody.The radiant emittance of a blackbody having an absolute temperature of"T" is governed by the Stefan-Boltzmann relationship, that is, the totalpower emitted per unit area of a blackbody is equal to σ·T⁴, where σ isStefan's constant. Thus, both the spectrum of the radiated energy andthe total intensity of the radiated energy are related to thetemperature of the radiating body. The temperature of an object cantherefore be determined by observing either the wavelength dependence ofthe energy radiated by the object, or by measuring the total intensityradiated from an area of known size on the surface of the object.Particularly preferred embodiments of the present invention are used tomonitor the temperature of glass substrates, which have emissivities ofapproximately one. Thus, the temperature of a glass substrate canreadily be determined from experimental observations of either thespectral energy distribution of the blackbody radiation or the totalintensity of the blackbody radiation.

FIG. 1 illustrates a preferred embodiment of the present invention. Theillustrated system is configured so that thin film deposition can begin,with a deposition substrate 10 disposed on the surface of a susceptor 12and adjacent and parallel to an RF (radio frequency) electrode 14. Thedeposition substrate 10 may be a piece of glass being processed to forma liquid crystal display device. In the illustrated embodiment, the RFelectrode 14 is coupled to an RF power supply 15. During deposition, anRF field is maintained between the RF electrode 14 and the susceptor 12to excite a plasma in the region 16 above the deposition substrate 10.Molecular gases are excited in the plasma region 16 and subsequently thegases or portions of the gases deposit on the surface of the depositionsubstrate 10 to form thin film layers on the substrate. The thin filmdeposition system preferably employs a massive aluminum block as thesusceptor 12 to heat the substrate 10 to an appropriate temperature fordeposition to occur. Preferably, the temperature of the susceptor 12 isheated to a deposition temperature of between about 300° C. to 400° C.and the susceptor temperature is maintained to within about ±1° C.

In the course of operation, new deposition substrates 10 are transportedfrom a preheat chamber (not shown) to the susceptor 12 by a transportrobot. The temperature of the deposition substrate 10 falls as thesubstrate is transported from the preheat chamber to the surface of thesusceptor 12 and the deposition substrate 10 must be (re-)heated to anappropriate temperature before deposition can begin. Deposition systemsin accordance with the present invention include a temperaturemeasurement system for monitoring the temperature of the depositionsubstrate 10. In this way, the substrate can reliably be heated to anappropriate deposition temperature before a deposition process begins.

Objects at room temperature, or at the moderately elevated temperaturesat which the substrate 10 and susceptor 12 are maintained during thinfilm deposition, emit blackbody radiation primarily in the infraredregion. In other words, both the deposition substrate 10 and thesusceptor 12 will emit optical radiation having a continuous spectrum ofwavelengths, the majority of which are within the infrared region. Athin film deposition system in accordance with the present inventionpreferably measures the temperature of the deposition substrate 10 atinfrared wavelengths. The system illustrated by FIG. 1 includes atemperature measurement system having an infrared detector 18 disposedaway from both the susceptor 12 and the deposition substrate 10. In theillustrated embodiment, an optical path extends from the backside of thedeposition substrate 10, through a channel 20 formed in the susceptor12, through an infrared window 22, off of a mirror 24 and to theinfrared detector 18. Preferably, the temperature measurement region 26on the backside of the deposition substrate 10 and the channel 20 formedin the susceptor 12 are made small so as to limit the amount of coolingthat occurs on the deposition substrate 10 due to the presence of thechannel 20 in the susceptor 12. For example, the temperature measurementregion 26 may be circular with a diameter of less than one quarter ofone inch. In practice, the allowable size of the temperature measurementregion 26 varies depending on the temperature sensitivity of thedeposition process and the thermal conductivity of the depositionsubstrate 10, and will further depend on how much viewing area is neededto reliably measure the temperature of the backside of the depositionsubstrate.

In particularly preferred embodiments of the present invention, a hollowlightguide 28 extends from the temperature measurement region 26 to theinfrared window 22. The walls of the tube-like lightguide 28 preferablydo not contact the walls of the susceptor which form the channel 20. Anylevel of separation reduces the efficiency of heat transfer between thesusceptor 12 and the lightguide 28, allowing the lightguide to remain ata lower temperature than the susceptor itself. The cooler walls of thelightguide 28 act to shield the detector from the radiation emitted bythe susceptor and to thus limit the level of background radiationcollected by the detector 18. To reduce the temperature of the walls ofthe lightguide 28 and consequently reduce the level of backgroundradiation incident on the detector, the lightguide 28 is preferablyformed separate from the susceptor 12. A loose mechanical fit betweenthe susceptor 12 and the lightguide 28 reduces the efficiency of thethermal conduction between the susceptor 12 and the lightguide 28,thereby reducing the temperature of the walls of the lightguide 28.

FIG. 2 illustrates a portion of the junction between the susceptor 12and the lightguide 28 for a preferred embodiment of the presentinvention. A ledge 30 extends from the inner walls 32 of the susceptor12 adjacent to the temperature measurement region 26. The ledge 30serves to keep the end of the lightguide 28 from touching the backsidesurface of the deposition substrate, thereby reducing the amount of heattransfer between the backside of the deposition substrate 10 and thelightguide 28. Use of the ledge 30 reduces the risk of developing a coldspot on the deposition substrate 10 produced by the lightguide 28contacting the deposition substrate and allowing heat to flow along thelightguide 28. The ledge 30 is preferably made thin to limit the amountof background radiation imaged from the ledge 30 onto the infrareddetector 18. There are, of course, other ways of maintaining thepreferred separation between the deposition substrate 10 and thelightguide 28.

In the embodiment illustrated in FIG. 1, infrared radiation from thebackside of the deposition substrate passes through an infrared window22 before it reaches the detector 18. The primary function of theinfrared window 22 is to provide an optical path through the chamberwall for the pyrometry measurement so that the infrared detector 18 canbe located outside the vacuum environment of the deposition chamber.Accordingly, a vacuum seal is maintained around the window 22, which hassufficient mechanical strength to withstand the vacuum pumping process.It is preferred that the window be highly transmissive at infraredwavelengths, and that the window have a substantially flat transmissioncurve for infrared radiation. It is possible to measure the transmissioncurve and to adjust the calculation of the temperature from theintensity of the blackbody radiation on the basis of the transmissioncurve. Since calibration of the temperature measurement system may beaccomplished using empirical methods, the effects of the transmissioncurve on the intensity can be measured when calibrating the system. Anumber of materials are available which have appropriate characteristicsto serve as the infrared window 22, i.e., the materials are transmissivein a band between about four microns and fourteen microns and haverelatively flat transmission spectra. One suitable material for theinfrared window is AMTIR-1, which is a germania glass manufactured byAmorphous Materials, Inc., of Garland Tex.

FIG. 1 shows that the lightguide 28 extends from the backside of thedeposition substrate 10 to the infrared window 22. Preferably, the sizeof the infrared window 22 is chosen so that the lightguide 28 slidesover the end of the infrared window 22. For example, the infrared windowmay preferably be machined down to have a circular cross section whichfits within the inner diameter of the lightguide 28. An embodiment ofthe infrared window 22 is shown in the partial assembly illustrated byFIG. 3. In particular, FIG. 3 shows an infrared window 22 in a vacuummount used to form a vacuum seal between the infrared window 22 andvacuum wall 40 which faces the interior of the deposition chamber. Thevacuum seal is formed by compressing an O-ring 42 fitted around theinfrared window 22 and disposed between the vacuum wall 40 andcompression wall 44. As the bolts 46 are tightened, the O-ring 42compresses to form a vacuum joint with the vacuum wall 40. At the sametime, the O-ring spreads inward to form a vacuum-tight joint with thelarger diameter portion of the infrared window 22. Other of the wellknown vacuum feed-through joints might be used to make this vacuum seal,as well.

FIG. 3 also shows the infrared window 22 mated with the lightguide 28.In this preferred embodiment of the present invention, the infraredwindow is machined so that a portion 48 of the window 22 adjacent to oneend has a sufficiently small diameter that the window can fit inside ofthe lightguide 28. A spring 50 of an appropriate inner diameter isdisposed about the small diameter portion 48 so that it makes contactwith the shoulder 52 of the window and with the end of the lightguide28. The spring 50 serves to keep tension on the lightguide so that itstays seated on the ledge 30 in the assembly illustrated in FIG. 2.

Referring again to FIG. 1, the infrared radiation from the backside ofthe deposition substrate preferably passes through the lightguide 28,through the infrared window 22, and on towards an infrared detector 18.Due to the restricted space available in the typical deposition system,it may be necessary to direct the optical path in such a way as to allowthe infrared detector 18 to be placed in the available space. In theembodiment illustrated in FIG. 1, a mirror 24 directs toward theinfrared detector 18 the infrared radiation which passes through theinfrared window 22. Any metal mirror can be used to direct the infraredradiation, but first surface gold mirrors are particularly preferredbecause of their very high reflectivity and the extreme flatness oftheir reflectance spectrum in the infrared. Under some circumstances, itmay be desirable to use a spherical or parabolic mirror instead of theflat mirror shown at 24. For example, a focusing mirror can be used toimage the temperature measurement region 26 onto the surface of theinfrared detector 18. Such an arrangement can be used to decrease thebackground radiation incident into the detector 18 if the appropriatespatial filtering is done to restrict the amount of radiation collectedby the focusing mirror.

In preferred embodiments of the present invention, the vacuum wall 40(see FIG. 3) is cooled to keep the infrared window from heating to anunacceptable temperature. In practice, the O-ring 42 is preferablymaintained at a temperature below 200° C., which means that the O-ringtypically has more stringent temperature requirements than the infraredwindow. A preferred cooling method is to direct water to the outside ofthe wall through cooling channels formed in or adjacent to the wall, asis generally practiced in the art.

An infrared detector 18 in accordance with the present invention may beone of a number of different types. A preferred embodiment of thepresent invention uses a Model OS62-MVC thermopile detector from OmegaCorporation, of Stamford, Conn. Thermopile detectors tend to beinexpensive, have relatively flat response curves, and are relativelyfast. Thermopiles derive these properties from the fact that thethermopile consists of an array of thermocouples disposed on a highemissivity substrate having a low thermal mass. Accordingly, thethermopile detector may be used for time varying and differentialmeasurements in the present invention.

Output from the infrared detector 18 is provided to a computer 19 thatpreferably calculates a temperature on the basis of the measuredintensity of infrared radiation. In addition, the output signal from theinfrared detector may be coupled to a control system within the computer19 which controls the deposition system in such a way that a depositionprocess does not begin until after the deposition substrate 10 hasreached the target temperature for thin film deposition.

Glass deposition substrates, such as those used in forming liquidcrystal display devices, are good candidates for optical pyrometersbecause, at typical thin film deposition temperatures, glass is a goodblackbody. The fact that glass is a good blackbody at these temperaturesis important for two reasons. First, it means that the glass has arelatively high emissivity within major portions of the radiationdistribution described by Planck's law and radiates substantially inaccordance with the blackbody radiation laws discussed above. Anapparent temperature may therefore be calculated from the intensity ofthe blackbody radiation to determine to a close approximation the actualtemperature of the substrate. Second, the fact that the glass is a goodblackbody is important because it means that the glass will be opaquethroughout most of the range of radiation wavelengths produced by thattemperature of blackbody. Thus, the glass will not directly transmit theinfrared tail of the intense blackbody radiation emitted by the plasma,and will act to shield the infrared detector from the plasma radiation.Because the glass is opaque through a range of wavelengths in theinfrared, the infrared detector can image the backside of the glassdeposition substrate within that range of wavelengths withoutcompensating for the background radiation emitted by the plasma. Thus,it may be preferable to utilize a detector that is sensitive only withina narrow window of infrared wavelengths. For example, it may be usefulto select a detector that is sensitive to wavelengths longer than about5 μm. Because the amount of emitted blackbody radiation falls offsubstantially at longer wavelengths, it is typically acceptable toselect a detector that is sensitive within a range of infraredwavelengths which might be, for example, between the wavelengths of 5 μmto 14 μm. In the alternative, a cutoff or bandpass filter may bedisposed along the detector's optical path to limit the wavelengthsensitivity of the system.

The thin film deposition environment contains a number of blackbodyemitters which produce background radiation signals that must becompensated for to measure the temperature of the glass depositionsubstrate by pyrometry. Primary among the blackbody sources in thedeposition environment is the plasma itself, which has an effectiveblackbody temperature on the order of thousands of degrees and emitsbroadband radiation including the entire optical and infrared ranges.The other major blackbody emitter is the susceptor 12 on which thedeposition substrate 10 is disposed. Because the susceptor 12 ismaintained at the target temperature for the deposition substrate 10, itwill have a blackbody radiation spectrum very similar to that of theglass substrate. The plasma will be the dominant radiation source duringdeposition, however, because the absolute temperature of the plasma isseveral times that of the susceptor, which likely means that the plasmais the "brightest" blackbody emitter in the deposition environment.

To eliminate the plasma radiation from the field of view of the infrareddetector, it is preferable to view the deposition substrate from thebackside of the substrate. In this configuration, the depositionsubstrate can be used to shield the infrared detector from the plasmaduring the deposition process and from the RF electrode, which has atemperature of about 80° C., when no RF plasma is present. The substratewill be an effective shield only for those wavelengths at which thesubstrate is essentially opaque. For example, the glass typically usedfor producing liquid crystal displays is substantially transparent atvisible wavelengths, and remains somewhat transparent for wavelengths aslong as 4-4.5 μm. Thus, a glass deposition substrate will typically onlybe an effective radiation shield at wavelengths longer than about 5 μm.At shorter wavelengths, too much radiation from the plasma would betransmitted through a glass substrate to make reliable pyrometrictemperature readings.

A second advantage to observing the backside of the deposition substrateis that the optical characteristics of the backside of the depositionsubstrate will not change appreciably during the deposition process. Bycontrast, the front surface of the deposition substrate will undergochange throughout the deposition process as increasing thicknesses offilm are deposited on the surface of the substrate. Thus, thetemperature of the substrate may be more simply determined from anobservation of the backside of the deposition substrate, as compared toa measurement made from the frontside of the deposition substrate.

Viewing the deposition substrate from the backside and restricting thewavelength of the measurement to a region at which the depositionsubstrate is substantially opaque reduces the level of the backgroundsignal associated with the radiation from the plasma. However, in thetypical configuration of a thin film deposition system, the backside ofthe deposition substrate 10 is disposed on a solid metal susceptor 12.The susceptor 12 is generally maintained at a temperature suitable forthin film deposition, which is typically between about 300° C. and 400°C. These temperatures are too high to operate most infrared detectors,including both semiconductor and pyroelectric detectors. Moreover, ifthe detector were maintained at a temperature near the temperature beingmeasured, the blackbody radiation of the detector itself would likely betoo high to allow accurate measurements to be made. It is thereforepreferable to maintain the infrared detector distant from the body ofthe susceptor while still viewing the backside of the depositionsubstrate.

In preferred embodiments of the present invention, the temperature ofthe deposition substrate is viewed through a channel or hole in thesusceptor. In this way, the infrared detector can be disposed distantfrom deposition substrate and the temperature of the substrate can beviewed from a distance. Preferably, the infrared radiation emitted bythe backside of the deposition substrate 10 is channeled or guided tothe infrared detector 18 through an optical path designed to maximizethe radiation collected from the backside of the deposition substrate 10and to minimize the amount of background radiation emitted by thesusceptor 12 that is incident on the detector 18. For example, thelightguide 28 may be used to channel infrared radiation through thesusceptor 12 from the backside of the deposition substrate 10. Thelightguide 28 is preferably sufficiently smooth on its inner surface sothat relatively little of the radiation emitted by the depositionsubstrate 10 is absorbed or reflected back toward the depositionsubstrate.

The lightguide 28 may be a ceramic tube and, in particular, may be analumina tube. Alumina is a suitable material for use because it isstable over a wide range of temperatures and environmental conditionsand because its elemental constituents, aluminum and oxygen, are foundelsewhere in the processing environment. For example, the susceptor 12is typically formed from aluminum, and glass deposition substratestypically include oxygen as an elemental constituent. It is generallyadvisable to not introduce different types of material into the thinfilm deposition environment to avoid the dangers of contamination orundesired reaction. Potential contamination problems may ariseunexpectedly due to the harsh chemical environment of the depositionchamber and because of the very energetic reactions facilitated by theplasma. It is therefore simplest to only use materials that are alreadypresent in the deposition chamber environment so that no newcontamination problems arise.

Because it is used in the chemically reactive environment of thedeposition chamber, the emissivity of the lightguide may vary over time.Process gases from the plasma region may reach the inner surface of thelightguide and react with the lightguide material, even if the processgases are present only in greatly reduced quantities. Thus, a variety ofchemical reactions may occur over time, altering the chemicalcomposition of the lightguide and consequently altering the opticalproperties of the lightguide. For the purposes of the pyrometric systemdescribed herein, one of the most important of the optical propertiesthat might change over time is the emissivity of the lightguide. Forexample, if the lightguide were formed from aluminum, prolonged exposureto some of the process gases used in thin film deposition might causethe inner surface of the lightguide to be converted into aluminum oxide.Aluminum oxide typically has a higher emissivity than does a polishedpiece of aluminum. Additionally, the chemical reaction which convertsaluminum to aluminum oxide might cause the inner surface of thelightguide to become rougher, increasing the scattering of light fromthe walls of the lightguide. Similar chemical reactions or physicalroughening may occur with other lightguide materials, as well.

If the optical properties of the lightguide, or any other opticalcomponent along the optical path from the backside of the depositionsubstrate to the infrared detector, vary over time, it may be preferableto periodically recalibrate the temperature measurement system.Calibration may be accomplished in a number of different ways. Adeposition substrate may be fitted with a thermocouple or otherthermometer to measure the actual temperature of the substrate forcomparison with the pyrometric determined temperature. A different sortof recalibration may be performed to allow the pyrometric temperaturemeasurements to be made as a differential measurement. For example, thebackground temperature measurement might be made by measuring theintensity of the background radiation when a "cold" glass substrate ismoved into place on the susceptor. For the purposes of calibrating thetemperature measurement system, a "cold" glass substrate is one having asufficiently low temperature that the blackbody radiation from thesubstrate is a negligible proportion of the total intensity collected bythe infrared detector. In practice, a room temperature substrate may besufficiently cold to perform background measurements. In such abackground measurement, the primary purpose of the cold glass substrateis to shield the infrared detector from any blackbody source disposedabove the usual substrate position. Either the level of backgroundradiation or the apparent background temperature can be used as abackground radiation reference signal. The background reference signalis then stored in a memory either within the calculation electronics orin the deposition system's computer 19. Subsequent pyrometricmeasurements are made relative to the previously determined backgroundradiation reference signal. In other words, the background radiation is"subtracted off" from the pyrometric measurements. The backgroundmeasurement can be made as often as is necessary to maintain theaccuracy of the temperature measurement.

Both aluminum and alumina may be suitable lightguide materials, butthere are tradeoffs associated with the choice of one material or theother. For example, an aluminum tube having a highly polished innersurface is highly reflective and consequently is expected to have asubstantially lower emissivity than an alumina tube. The use of analuminum tube would therefore decrease the amount of background signalproduced by the blackbody radiation from the walls of the lightguide.However, an alumina lightguide has greater chemical stability, and it isless likely that the optical properties of the lightguide will vary overtime. Accordingly, the choice of lightguide material will often dependon the exact nature of the chemicals used for the thin film deposition.As discussed above, it is preferable that the lightguide be formed of amaterial already present in the deposition environment. It is, ofcourse, possible to use a different material for the lightguide, if thematerial is chosen so as to not produce unacceptable contamination ofthe thin films deposited in the system.

A second design consideration in choosing the lightguide material isthat the walls of the lightguide will conduct heat away from thedeposition substrate. Such heat conduction can cause a local cold spotto develop on the surface of the deposition substrate. The surface ofsuch a cold spot might have a temperature slightly lower thansurrounding regions of the deposition substrate, and might producemeasurably different electrical properties in the films deposited inthat region. Unacceptable uniformity variations in the deposited thinfilms might result if the temperature drop at the surface of thesubstrate were sufficient to produce measurable differences in theproperties of thin films deposited in that region. To reduce the amountof heat conducted away from the deposition substrate, the walls of thelightguide are preferably made as thin as possible so that thelightguide has as low of a thermal conductivity as possible, while stillmaintaining sufficient mechanical strength to survive vacuum pumpingcycles within the deposition environment. To further reduce the amountof heat flow away from the surface of the deposition substrate, it isdesirable to form the lightguide from a low thermal conductivitymaterial. From this point of view, a ceramic tube is more desirable thanan aluminum tube. However, because thermal conductance is a function ofthe cross sectional area of the lightguide and because an aluminum tubecan have substantially thinner walls than a ceramic tube, an aluminumtube can be made to have a thermal conductance approaching that of aceramic tube.

The lightguide is preferably physically isolated from the susceptoralong almost all of its length to reduce the background radiation fromthe walls of the tube. Under some circumstances, however, the lightguideitself may be a part of the susceptor 12. For example, the susceptor 12may be formed from aluminum, the optical emissivity of the aluminuminner surface of the channel through the susceptor may not varyunacceptably with time, and the signal output from the depositionsubstrate may be sufficiently strong so that the blackbody radiation ofthe susceptor can be adequately subtracted. Considerable accuracy couldbe lost using such a configuration, however, and it is believed thatsuch a configuration will not produce satisfactory temperature trackingcapability in some circumstances.

While the present invention has been described with reference tospecific preferred embodiments thereof, it will be understood by thoseskilled in this art that various changes may be made without departingfrom the true spirit and scope of the invention. In addition, manymodifications may be made to adapt the invention to a given situationwithout departing from its essential teachings.

What is claimed is:
 1. A method of depositing thin films of materialcomprising:transporting a deposition substrate to a surface of a heatedsusceptor; heating said deposition substrate to a target temperature;passing infrared radiation from said deposition substrate through ahollow lightguide disposed within a channel formed in said susceptor;collecting said infrared radiation within a range of infraredwavelengths from said deposition substrate; determining an apparenttemperature of said deposition substrate from said infrared radiationcollected from said deposition substrate; and depositing a layer ofmaterial on said deposition substrate.
 2. The method of claim 1 whereinsaid hollow lightguide comprises a cylindrical tube having an innersurface that is highly reflective at infrared wavelengths.
 3. The methodof claim 1 wherein the determining an apparent temperaturecomprises:measuring the intensity of said infrared radiation from saiddeposition substrate; and calculating said apparent temperature on thebasis of the blackbody radiation characteristics of said depositionsubstrate.
 4. The method of claim 1 wherein depositing a layer ofmaterial on said deposition substrate is controlled to begin when saidapparent temperature is equal to said target temperature.
 5. The methodof claim 1 further comprising:transporting a cold deposition substrateto said surface of said susceptor; measuring a level of backgroundradiation before said cold deposition substrate is heated; storing abackground reference signal; and making successive temperaturemeasurements relative to said background reference signal.
 6. The methodof claim 5 wherein the collecting infrared radiation comprises:passingsaid infrared radiation from said deposition substrate through aninfrared window; and directing said infrared radiation from saiddeposition substrate to an infrared detector.
 7. The method of claim 6wherein the passing infrared radiation from said deposition substratethrough a hollow lightguide further comprises shielding said infrareddetector from a background radiation signal produced by said susceptor.8. A method of depositing thin films of material comprising the stepsof:transporting a deposition substrate to a surface of a heatedsusceptor; heating said deposition substrate to a target temperature;collecting infrared radiation within a range of infrared wavelengthsfrom said deposition substrate; directing said infrared radiation fromsaid deposition substrate to an infrared detector: shielding saidinfrared detector from a background radiation signal produced by saidsusceptor: determining an apparent temperature of said depositionsubstrate from said infrared radiation collected from said depositionsubstrate; and depositing a layer of material on said depositionsubstrate.
 9. The method of claim 8 wherein the step of determining anapparent temperature comprises the steps of:measuring the intensity ofsaid infrared radiation from said deposition substrate; and calculatingsaid apparent temperature on the basis of the blackbody radiationcharacteristics of said deposition substrate.
 10. The method of claim 8wherein the step of depositing a layer of material on said depositionsubstrate is controlled to begin when said apparent temperature issubstantially equal to said target temperature.
 11. The method of claim8 further comprising the steps of:transporting a cold depositionsubstrate to said surface of said susceptor; measuring a level ofbackground radiation before said cold deposition substrate issubstantially heated; storing a background reference signal; and makingsuccessive temperature measurements relative to said backgroundreference signal.
 12. The method of claim 11 wherein the step ofcollecting infrared radiation comprises the steps of:passing saidinfrared radiation from said deposition substrate through a channelformed in said susceptor; and passing said infrared radiation from saiddeposition substrate through an infrared window.
 13. A method ofdepositing thin films of material comprising:transporting a depositionsubstrate to a surface of a heated susceptor; heating said depositionsubstrate to a target temperature; passing infrared radiation from saiddeposition substrate through a channel formed in said susceptor; passingsaid infrared radiation from said deposition substrate through aninfrared window; directing said infrared radiation from said depositionsubstrate to an infrared detector; determining an apparent temperatureof said deposition substrate from said infrared radiation collected fromsaid deposition substrate; and depositing a layer of material on saiddeposition substrate.
 14. The method of claim 13 wherein the determiningan apparent temperature comprises:measuring the intensity of saidinfrared radiation from said deposition substrate; and calculating saidapparent temperature on the basis of the blackbody radiationcharacteristics of said deposition substrate.
 15. The method of claim 13wherein the depositing of a layer of material on said depositionsubstrate is controlled to begin when said apparent temperature is equalto said target temperature.
 16. The method of claim 13 furthercomprising:transporting a cold deposition substrate to said surface ofsaid susceptor; measuring a level of background radiation before saidcold deposition substrate is heated; storing a background referencesignal; and making successive temperature measurements relative to saidbackground reference signal.
 17. The method of claim 16 furthercomprising shielding said infrared detector from a background radiationsignal produced by said susceptor.
 18. The method of claim 16 furthercomprising passing said infrared radiation from said depositionsubstrate through a hollow lightguide to shield said infrared detectorfrom a background radiation signal produced by said susceptor.
 19. Themethod of claim 18 wherein said lightguide comprises a cylindrical tubehaving an inner surface that is highly reflective at infraredwavelengths.
 20. A method of depositing thin films of materialcomprising:transporting a deposition substrate to a surface of a heatedsusceptor; heating said deposition substrate to a target temperature;collecting infrared radiation within a range of infrared wavelengthsfrom said deposition substrate; directing said infrared radiation fromsaid deposition substrate to an infrared detector, wherein said infrareddetector is sensitive to infrared wavelengths longer than a systemcutoff wavelength and wherein said infrared detector is insensitive tooptical radiation having wavelengths shorter than or equal to saidsystem cutoff wavelength, wherein said deposition substrate istransmissive to optical radiation having wavelengths shorter than saidsystem cutoff wavelength and said deposition substrate is opaque tooptical radiation having wavelengths equal to or longer than said systemcutoff wavelength when said deposition substrate is at a depositiontemperature; determining an apparent temperature of said depositionsubstrate from said infrared radiation collected from said depositionsubstrate; and depositing a layer of material on said depositionsubstrate.
 21. The method of claim 20 wherein the determining anapparent temperature comprises:measuring the intensity of said infraredradiation from said deposition substrate; and calculating said apparenttemperature on the basis of the blackbody radiation characteristics ofsaid deposition substrate.
 22. The method of claim 20 wherein thedepositing a layer of material on said deposition substrate iscontrolled to begin when said apparent temperature is equal to saidtarget temperature.
 23. The method of claim 20 furthercomprising:transporting a cold deposition substrate to said surface ofsaid susceptor; measuring a level of background radiation before saidcold deposition substrate is heated; storing a background referencesignal; and making successive temperature measurements relative to saidbackground reference signal.
 24. The method of claim 23 wherein thecollecting of infrared radiation comprises:passing said infraredradiation from said deposition substrate through an infrared window; anddirecting said infrared radiation from said deposition substrate to aninfrared detector.
 25. The method of claim 24 further comprisingshielding said infrared detector from a background radiation signalproduced by said susceptor.
 26. The method of claim 24 furthercomprising passing said infrared radiation from said depositionsubstrate through a hollow lightguide to shield said infrared detectorfrom a background radiation signal produced by said susceptor.
 27. Themethod of claim 26 wherein said lightguide comprises a cylindrical tubehaving an inner surface that is highly reflective at infraredwavelengths.
 28. A method of depositing thin films of materialcomprising the steps of:transporting a cold deposition substrate to asurface of a heated susceptor: measuring a level of backgroundradiation; storing a background reference signal; heating saiddeposition substrate to a target temperature; collecting infraredradiation within a range of infrared wavelengths from said depositionsubstrate, wherein said step of collecting infrared radiation comprisesthe steps of:passing said infrared radiation from said depositionsubstrate through a channel formed in said susceptor; passing saidinfrared radiation from said deposition substrate through an infraredwindow; and directing said infrared radiation from said depositionsubstrate to an infrared detector; shielding said infrared detector froma background radiation signal produced by said susceptor; determining anapparent temperature of said deposition substrate from said infraredradiation collected from said deposition substrate; making successivetemperature measurements relative to said background reference signal;and depositing a layer of material on said deposition substrate.
 29. Amethod of depositing thin films of material comprising the stepsof:transporting a cold deposition substrate to a surface of a heatedsusceptor; measuring a level of background radiation; storing abackground reference signal; heating said deposition substrate to atarget temperature; collecting infrared radiation within a range ofinfrared wavelengths from said deposition substrate, wherein said stepof collecting infrared radiation comprises the steps of:passing saidinfrared radiation from said deposition substrate through a channelformed in said susceptor; passing said infrared radiation from saiddeposition substrate through a hollow lightguide to shield an infrareddetector from a background radiation signal produced by said susceptor;passing said infrared radiation from said deposition substrate throughan infrared window; and directing said infrared radiation from saiddeposition substrate to said infrared detector; determining an apparenttemperature of said deposition substrate from said infrared radiationcollected from said deposition substrate; making successive temperaturemeasurements relative to said background reference signal; anddepositing a layer of material on said deposition substrate.
 30. Themethod of claim 29 wherein said lightguide comprises a cylindrical tubehaving an inner surface that is highly reflective at infraredwavelengths.